Flotation cells are often used nowadays in order to extract ores from ore-containing bulk material removed during mining. A flotation cell is supplied with an ore-containing pulp, that is to say a suspension consisting of water, of ground rock and of ground ore. All insoluble solids in the pulp form the solid fraction of the pulp which is usually around at least 30% by mass. As a rule, the solid fraction is higher than 30% by mass, in particular around at least 40% by mass, since a flotation cell can be operated appropriately in an efficient way only beyond these solid fractions.
As a result of the addition of appropriate chemicals, the ore particles in the suspension are hydrophobized. A gas, usually air, is injected into the flotation cell. The formation of small bubbles occurs in the pulp. The hydrophobized ore particles adhere to the likewise hydrophobic bubbles and are transported to the surface. A foam having a high ore content is formed there. This foam is removed from the pump via foam run-off gutters and is further processed.
The disadvantage of this solution is that the bubbles which occur are difficult to control. Furthermore, the time until the ore-rich foam passes from the pulp reservoir into the foam run-off gutter is relatively long because of the high viscosity of the pulp. The result of this is that ore particles adhering to the bubbles may be lost again due to interactions of air bubbles with one another. The output of the flotation cell is thereby reduced.
In an alternative variant of ore extraction, non-magnetic ores are extracted from a pulp with the aid of magnetic particles. These usually have a higher output of the ore from the pulp than the conventional flotation method by means of a flotation cell. In this alternative variant, the properties, in particular the surface properties, of magnetic particles are set in such a way that these particles accrete selectively onto the ore to be extracted. By means of a magnetic field, the ore adhering to the magnetic particles can then be removed from the pulp.
Methods of this type are known, for example from U.S. Pat. No. 4,657,666 and U.S. Pat. No. 3,926,789.
The disadvantage of these methods is that large quantities of magnetic particles have to be provided in relation to the quantity of ore to be extracted. This requires a high outlay in logistical terms and may lead to the unprofitability of the plant.
To make the problem even clearer, the following example is given. In a typical mine, as a rule, several 1000 t of bulk material are worked through per hour. The valuable substance, for example ore, has to be withdrawn from this removed bulk material. The bulk material often contains, on average, approximately 1% to 2% of ore to be extracted. Since the ratio of ore and magnetic particles in the extraction of ore by means of magnetic separation is approximately of the same order of magnitude, about 10 t to 100 t of magnetic particles per hour are therefore required, that is to say 240 t to 2400 t per day. This means that the mine requires approximately 10 to 100 truckloads of magnetic particles per day so that the ore extraction process can be maintained. In addition, environmental pollution is caused by the “spent” ore particles. Considerable costs are thereby incurred.